The treatment of industrial water systems, particularly cooling water systems, has over the past 25 years been subject to significant changes. The most prominently recognized treatment for cooling water systems came in the form of Betz Laboratories, Inc.'s Dianodic.RTM. and Zinc-Dianodic product lines. These lines made use of the exceptional capacity of the chromate component in inducing the formation of a passive oxide film, believed to be primarily gamma-ferric oxide, on the metallic surfaces which provided protection against corrosion. The chromates, when used in conjunction with polyphosphates, zinc and in some cases orthophosphates, provided protection which until recently was basically unduplicatable with other treatments. In this regard, U.S. Pat. Nos. 2,711,391; 2,793,932; 2,848,299; 2,872,281; and 2,900,222 can be noted.
With the advent of Federal, State and Municipal environmental controls, however, chromate became suspect for its environmental impact on streams, ponds, lakes, etc., where it might be discharged. Some industries, particularly the petroleum refining, petrochemical, chemical and steel industries, because of each's awareness of the excellent passivation provided by the chromates through the formation of a passive oxide film, decided to continue to use the chromate treatments, with the attendant high capital expenditures for either chromate removal or recovery systems, or for disposing of reduced chromate obtained by the natural treatment of effluents. Other industries, on the contrary, have utilized treatments which avoided the use of chromates. Since chromates are superb corrosion inhibitors, systems such as cooling water systems, were maintained in the acid range, thereby avoiding the pH's where calcium carbonate and other salts normally form and precipitate.
With the prohibitions relative to the use of chromate, treatment programs were utilized which made use of the controlled precipitation of calcium phosphate and/or carbonate to form a protective barrier on the surface of the metallic parts (generally ferrous metals) to provide corrosion protection through cathodic action. These programs utilized orthophosphates, polyphosphates, phosphonic acid compounds and their salts. Although these programs were reasonably successful, they did not provide the protection established by the passive oxide film induced by chromate treatments. Moreover, these programs, because of the need for narrow ranges of pH control, were virtually unforgiving, i.e., if the concentration of the calcium ion, the phosphate or phosphonate were excessive for a particular pH, uncontrolled precipitation occurred, which on many occasions blocked the metal pipes which the programs were attempting to protect. Control of the operating parameters and conditions was extremely critical.
Typical of this approach is U.S. Pat. No. 4,209,398 (Ii et al). In accordance with this disclosure a polymer having an ethylenically unsaturated bond and having one or more carboxyl radicals, wherein at least a portion of the carboxyl radicals are modified, is combined with one or more compounds selected from the group consisting of inorganic phosphates, phosphonic acids, organic phosphoric acid esters and polyvalent metal salts. This treatment is then added to the water system. Specifically exemplified polymers include terpolymers such as, inter alia, a terpolymer of the sodium salt of acrylic acid/2-hydroxyethyl methacrylate/and methyl acrylate. The disclosed treatment may also comprise a chromate corrosion inhibitor.
These treatments, although the best available at the time, not only reduced heat transfer because of the deposited materials, but also negatively affected production because they impeded flow, for example, of the cooling water. Each of these factors had a direct effect on energy costs, since more energy was required to provide commensurate production to that achieved when chromate was used. One such chromate-free treatment is disclosed in U.S. Pat. No. 3,837,803 (Vogt et al).
In allowed U.S. Application Ser. No. 101,658 (May et al), of common assignment herewith, a treatment is disclosed which successfully establishes the much desired but elusive passive oxide film (also believed to be a gamma-ferric oxide film) on ferrous-based metallic surfaces in contact with aqueous systems, particularly cooling water systems. The disclosed treatment comprises, inter alia, orthophosphate, a copolymer and an organo-phosphonate compound, the copolymer being composed primarily of acrylic moieties and hydroxylated lower alkyl acrylate moieties. This particular copolymer is disclosed in U.S. Pat. No. 4,029,577 (Godlewski et al). Previous to the development of this treatment, only chromate treatments had been effective in producing the protective oxide film.
Although the treatment disclosed in Application Ser. No. 101,658 provides a major breakthrough in the art, since high levels (about 10-20 ppm) of orthophosphate are typically utilized, iron induced fouling has been observed in several field applications of the treatment in those instances where iron containing well water was used as a makeup source.
This iron poses a unique problem in that it is soluble in the well and is rapidly oxidized to an insoluble form, probably an hydroxide, in the cooling system. This hydroxide forms a very surface active precipitate which is the root of the fouling problem. This fouling is particularly severe in the presence of phosphate based cooling water treatments. The deposits in the phosphate systems contain calcium, iron and phosphate. It is not known whether the chemistry of the deposit is calcium phosphate and iron hydroxide or a complex iron/calcium/phosphate salt. Nevertheless, the inclusion of phosphate in the deposit is particularly damaging because it results in a deficiency of soluble phosphate for corrosion protection in addition to promoting fouling.
Accordingly, an object of the present invention is to provide an improvement over the treatment disclosed in the above application, for use in those particular situations in which the water system to be treated contains iron species under deposit forming conditions.